This invention relates to a method and apparatus for manufacturing a cathode ray tube.
Generally, a panel or face plate and a funnel of a cathode ray tube are prepared independently and then fused together, usually with frit glass to form an envelope.
The steps of preparing the panel and the funnel will firstly be described. Firstly, the panel is applied with a phosphor film, an organic resin film, a metallic reflective film, i.e., a metal back and when desired, nonluminous black matrix film (hereinafter termed a BM film) and then the coated panel is baked in a furnace for burning off an organic substance utilized to coat the phosphor and the organic resin film. An electroconductive material such as graphite is applied to the inner surface of the funnel to form a conductive film.
A slurry method or air setting method is used to form a phosphor film on the inner surface of the panel, and an emulsion method, or a spray method is used to form the organic resin film. Such techniques have already been well known in the art as disclosed in "Color Television Screening by the Slurry Process" and "Emulsion Filming for Color Television Screens" both written by Theodore A. Saulnier, Jr. Electrochemical Technology Vol. 4, Nos. 1-2, p. 27-34, January-February, 1966.
After these treatments, the panel and funnel are fused together with frit glass in a furnace to form the envelope.
To simplify the manufacturing step in recent years, it has been proposed to execute the panel baking and fusion with frit glass by a single step as disclosed in Japanese Preliminary Publication of Patent Specification No. 123654/78 dated Oct. 28, 1978. We have found that such simplified method accompanies the following problems.
Thus, to decompose the organic material in the phosphor film during the panel baking step, substantially large quantity of oxygen is consumed and since air is supplied into the furnace only through the inlet port and the exit port of the furnace, the amount of oxygen supplied into the furnace is not sufficient.
With the prior art panel baking process only the panel is baked so that as the contact surface with air is large, a large quantity of oxygen consumption at the time of decomposing the organic substance causes no serious problem, whereas with the simplified process, under the assembled state of the panel and funnel, the area through which air is introduced into the assembly is limited to the area of the neck opening of the funnel.
In a 20 inches television picture tube, the ratio of the area of the funnel neck opening to the area of the panel is about 1/260. Thus, in the simplified method in which the panel baking and fusion of the panel and the funnel are performed simultaneously, the area of the funnel neck is too small to introduce a quantity of air sufficient to satisfy a large quantity of oxygen consumption at the time of decomposition of the organic substance. Usually, since the panel baking is effected while successively conveying a large number of tube assembly by a conveyor, a relatively long heating furnace with an inlet and an outlet at both ends is generally used. With such a furnace the ratio of the length of the furnace to the areas of the inlet and exit openings is large so that it is difficult to supply sufficient quantity of air into the heating portion of the furnace. For example, in a furnace having a length of 40 m and passed by 100 tube assemblies per hour, an oxygen concentration of 21.5% (calculated at 20.degree. C. at a time immediately after starting the operation) has decreased to 17% after 24 hours, whereas where the assembly is passed at a rate of 150 per hour the oxygen concentration decreased to 15.5%. The result of our experiment shows that the oxygen concentration decreases with time.
When the quantity of oxygen in the furnace becomes defficient as above described, the decomposition of the organic substance does not undergo completely, with the result that the organic substance remains as a carbonized state thus decreasing the brightness of the completed color picture tube. In addition, presence of not yet decomposed organic substance increases the quantity of impurity gasses in the tube thereby shortening the effective life of the tube and electro emission of electron guns. For this reason, where the panel baking is performed concurrently with panel-funnel fusion the quality of the product is degraded.
One may consider that such problem might be solved by enlarging the inlet and exit openings of the furnace. Such measure, however, not only makes it difficult to realize a desired schedule of rising and lowering the temperature in the furnace but also increases energy necessary to heat the furnace.
For this reason, according to the Preliminary Publication of Patent Specification cited above, a nozzle is inserted into the funnel opening of each tube assembly passed through the baking furnace so as to forcibly circulate the air in the tube. With this method, however, a nozzle attached to each tube assembly passed through the baking furnace admits air short of oxygen prevailing in the furnace into the tube assembly, so that it is impossible to introduce a sufficient amount of air into each tube assembly.